git-codereview: document mail -trybot flag
git-codereview: document mail -trybot flag

Change-Id: I86981168df8571e2408ef8bb5c5c0f21e0f3cca6
Reviewed-on: https://go-review.googlesource.com/24960
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

golang/review